EP0873617B1 - Key agreement and transport protocol with implicit signatures - Google Patents
Key agreement and transport protocol with implicit signatures Download PDFInfo
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- EP0873617B1 EP0873617B1 EP96944186A EP96944186A EP0873617B1 EP 0873617 B1 EP0873617 B1 EP 0873617B1 EP 96944186 A EP96944186 A EP 96944186A EP 96944186 A EP96944186 A EP 96944186A EP 0873617 B1 EP0873617 B1 EP 0873617B1
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- 238000000034 method Methods 0.000 claims description 32
- 238000004891 communication Methods 0.000 claims description 4
- 230000007774 longterm Effects 0.000 description 14
- 230000005540 biological transmission Effects 0.000 description 6
- 230000001010 compromised effect Effects 0.000 description 1
- 238000012790 confirmation Methods 0.000 description 1
- 230000002452 interceptive effect Effects 0.000 description 1
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L9/00—Cryptographic mechanisms or cryptographic arrangements for secret or secure communications; Network security protocols
- H04L9/08—Key distribution or management, e.g. generation, sharing or updating, of cryptographic keys or passwords
- H04L9/0816—Key establishment, i.e. cryptographic processes or cryptographic protocols whereby a shared secret becomes available to two or more parties, for subsequent use
- H04L9/0838—Key agreement, i.e. key establishment technique in which a shared key is derived by parties as a function of information contributed by, or associated with, each of these
- H04L9/0841—Key agreement, i.e. key establishment technique in which a shared key is derived by parties as a function of information contributed by, or associated with, each of these involving Diffie-Hellman or related key agreement protocols
- H04L9/0844—Key agreement, i.e. key establishment technique in which a shared key is derived by parties as a function of information contributed by, or associated with, each of these involving Diffie-Hellman or related key agreement protocols with user authentication or key authentication, e.g. ElGamal, MTI, MQV-Menezes-Qu-Vanstone protocol or Diffie-Hellman protocols using implicitly-certified keys
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L9/00—Cryptographic mechanisms or cryptographic arrangements for secret or secure communications; Network security protocols
- H04L9/32—Cryptographic mechanisms or cryptographic arrangements for secret or secure communications; Network security protocols including means for verifying the identity or authority of a user of the system or for message authentication, e.g. authorization, entity authentication, data integrity or data verification, non-repudiation, key authentication or verification of credentials
- H04L9/3247—Cryptographic mechanisms or cryptographic arrangements for secret or secure communications; Network security protocols including means for verifying the identity or authority of a user of the system or for message authentication, e.g. authorization, entity authentication, data integrity or data verification, non-repudiation, key authentication or verification of credentials involving digital signatures
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- G—PHYSICS
- G06—COMPUTING; CALCULATING OR COUNTING
- G06F—ELECTRIC DIGITAL DATA PROCESSING
- G06F7/00—Methods or arrangements for processing data by operating upon the order or content of the data handled
- G06F7/60—Methods or arrangements for performing computations using a digital non-denominational number representation, i.e. number representation without radix; Computing devices using combinations of denominational and non-denominational quantity representations, e.g. using difunction pulse trains, STEELE computers, phase computers
- G06F7/72—Methods or arrangements for performing computations using a digital non-denominational number representation, i.e. number representation without radix; Computing devices using combinations of denominational and non-denominational quantity representations, e.g. using difunction pulse trains, STEELE computers, phase computers using residue arithmetic
- G06F7/724—Finite field arithmetic
- G06F7/725—Finite field arithmetic over elliptic curves
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y04—INFORMATION OR COMMUNICATION TECHNOLOGIES HAVING AN IMPACT ON OTHER TECHNOLOGY AREAS
- Y04S—SYSTEMS INTEGRATING TECHNOLOGIES RELATED TO POWER NETWORK OPERATION, COMMUNICATION OR INFORMATION TECHNOLOGIES FOR IMPROVING THE ELECTRICAL POWER GENERATION, TRANSMISSION, DISTRIBUTION, MANAGEMENT OR USAGE, i.e. SMART GRIDS
- Y04S40/00—Systems for electrical power generation, transmission, distribution or end-user application management characterised by the use of communication or information technologies, or communication or information technology specific aspects supporting them
- Y04S40/20—Information technology specific aspects, e.g. CAD, simulation, modelling, system security
Definitions
- the present invention relates to key agreement protocols for transfer and authentication of encryption keys.
- the correspondents In a secret key cryptographic protocol, the correspondents share a common key that is secret to them. This requires the key to be agreed upon between the correspondents and for provision to be made to maintain the secrecy of the key and provide for change of the key should the underlying security be compromised.
- Public key cryptographic protocols were first proposed in 1976 by Diffie-Hellman and utilized a public key made available to all potential correspondents and a private key known only to the intended recipient.
- the public and private keys are related such that a message encrypted with the public key of a recipient can be readily decrypted with the private key but the private key cannot be derived from the knowledge of the plaintext, ciphertext and public key.
- Key establishment is the process by which two (or more) parties establish a shared secret key, called the session key.
- the session key is subsequently used to achieve some cryptographic goal, such as privacy.
- the number of message exchanges required between the parties is called the number of passes.
- a key establishment protocol is said to provide implicit key authentication (or simply key authentication) if one party is assured that no other party aside from a specially identified second party may learn the value of the session key.
- the property of implicit key authentication does not necessarily mean that the second party actually possesses the session key.
- a key establishment protocol is said to provide key confirmation if one party is assured that a specially identified second party actually has possession of a particular session key. If the authentication is provided to both parties involved in the protocol, then the key authentication is said to be mutual; if provided to only one party, the authentication is said to be unilateral.
- Examples include the Nyberg-Rueppel one-pass protocol and the Matsumoto-Takashima-Imai (MTI) and the Goss and Yacobi two-pass protocols for key agreement.
- the Nyberg-Rueppel protocol and the MTI protocol are described in EP 0 639 907 and MATSUMOTO, T., TAKASHIMA, Y., and IMAI, H.: On Seeking Smart Public-Key-Distribution Systems, The Transactions of the IECE of Japan, E69: 99-106, 1986.
- the Goss protocol is described in U.S. Patent No. 4,956,865.
- the Yacobi protocol is described in Y. Yacobi, "A Key Distribution Paradox," Advances in Cryptology, Crypto '90, Lecture Notes in Computer Science 537, Springer-Verlag, 1991, pp. 268 to 273.
- the prior proposals ensure that transmissions between correspondents to establish a common key are secure and that an interloper cannot retrieve the session key and decrypt the ciphertext. In this way security for sensitive transactions such as transfer of funds is provided.
- the MTI/AO key agreement protocol establishes a shared secret K, known to the two correspondents, in the following manner:-
- A In order to compute the key K, A must use his secret key a and the random integer x, both of which are known only to him. Similarly B must use her secret key b and random integer y to compute the session key K. Provided the secret keys a,b remain uncompromised, an interloper cannot generate a session key identical to the other correspondent. Accordingly, any ciphertext will not be decipherable by both correspondents.
- EP 0 639 907 describes a key agreement method.
- User A has a private key agreement key s A .
- a practical scenario where such an attack may be launched successfully is the following.
- B is a bank branch and A is an account holder. Certificates are issued by the bank headquarters and within the certificate is the account information of the holder.
- the protocol for electronic deposit of funds is to exchange a key with a bank branch via a mutually authenticated key agreement.
- B has authenticated the transmitting entity, encrypted funds are deposited to the account number in the certificate. If no further authentication is done in the encrypted deposit message (which might be the case to save bandwidth) then the deposit will be made to E's account.
- a pair of correspondents, 10,12 exchange information over a communication channel 14.
- a cryptographic unit 16,18 is interposed between each of the correspondents 10,12 and the channel 14.
- a key 20 is associated with each of the cryptographic units 16,18 to convert plaintext carried between each unit 16,18 and its respective correspondent 10,12 into ciphertext carried on the channel 14.
- a message generated by correspondent A, 10 is encrypted by the unit 16 with the key 20 and transmitted as ciphertext over channel 14 to the unit 18.
- the key 20 operates upon the ciphertext in the unit 18 to generate a plaintext message for the correspondent B, 12. Provided the keys 20 correspond, the message received by the correspondent 12 will be that sent by the correspondent 10.
- the system parameters for these protocols are a prime number p and a generator a of the multiplicative group Z p * .
- text A refers to a string of information that identifies party A. If the other correspondent B possesses an authentic copy of correspondent A's public key, then text A will contain A's public-key certificate, issued by a trusted center; correspondent B can use his authentic copy of the trusted center's public key to verify correspondent A's certificate, hence obtaining an authentic copy of correspondent A's public key.
- an interloper E wishes to have messages from A identified as having originated from E herself.
- E does not know the exponent ae, although she knows e.
- the correspondent B will assume that the message originates from E rather than A and use E's public key to generate the session key K.
- E also intercepts the message from B and uses his secret random integer e to modify its contents. A will then use that information to generate the same session key allowing A to communicate with B.
- the protocol is for parties A and B to establish a session key K.
- the protocols exemplified are role-symmetric and non-interactive.
- the protocols labelled First Protocol, Modified First Protocol, Second Protocol, Third Protocol, and Key Transport Protocol are presented to show general concepts, but these particular protocols are not part of the invention presently claimed.
- the system parameters for this protocol are a prime number p and a generator a of the multiplicative group Z p * .
- B will compute ⁇ s B ( p E ) r A which will not correspond with the transmitted value of r A . B will thus be alerted to the interloper E and will proceed to initiate another session key.
- Protocol 1 One draw back of the first protocol is that it does not offer perfect forward secrecy. That is, if an adversary leams the long-term private key a of party A, then the adversary can deduce all of A's past session keys.
- the property of perfect forward secrecy can be achieved by modifying Protocol 1 in the following way.
- step 1 A also sends ⁇ x 1 to B, where x 1 is a second random integer generated by A.
- B also sends ⁇ y 1 to A, where y 1 is a random integer.
- the second protocol improves upon the first protocol in the sense that if offers perfect forward secrecy. While it is still the case that disclosure of a private random integer x allows an adversary to learn the private key a, this will not be a problem in practice because A can destroy x as soon as she uses it in step 1 of the protocol.
- the second protocol is a three-pass protocol.
- the quantity s A serves as A's signature on the value ⁇ x
- This signature has the novel property that it can only be verified by party B. This idea can be generalized to all ElGamal-like signatures schemes.
- the first and second protocols above can be modified to improve the bandwidth requirements and computational efficiency of the key agreement.
- the modified protocols are described below as Protocol 1' and Protocol 2'. In each case, A and B will share the common key ⁇ s A s B .
- a and B thus share the common key but it will be noted that the signatures s A and s B need not be transmitted.
- a further protocol is available for parties A and B to establish a session key K.
- the system parameters for this protocol are a prime number p and a generator ⁇ for the multiplicative group Z p * .
- (r A , s A ) can be thought of as the signature of r x 1 with the property that only A can sign the message r x 1 .
- the protocols described above permit the establishment and authentication of a session key K. It is also desirable to establish a protocol in which permits A to transport a session key K to party B. Such a protocol is exemplified below.
- the above protocol may be modified to reduce the bandwidth by avoiding the need to transmit the signature S A as follows:
- All one-pass key transport protocols have the following problem of replay.
- a one-pass key transport protocol is used to transmit a session key K from A to B as well as some text encrypted with the session key K.
- E records the transmission from A to B. If E can at a later time gain access to B's decryption machine (but not the internal contents of the machine, such as B's private key), then, by replaying the transmission to the machine, E can recover the original text. (In this scenario, E does not learn the session key K).
- This replay attack can be foiled by usual methods, such as the use of timestamps.
- B has limited computational resources, in which it is more suitable at the beginning of each session, for B to transmit a random bit string k to A.
- the session key that is used to encrypt the text is then k ⁇ K, i.e. k XOR'd with K.
- the above protocol may also be implemented using a subgroup of Z p * .
- q will be a prime divisor of (p-1) and g will be an element of order p in Z p * .
- A's and B's public keys will be of the form g a , g b respectively and the short-term keys r a , r b will be of the form g x , g y .
- the signature components s A , s B are computed mod q and the session key K computed mod q as before.
- the shared secret is then g s A s B mod p.
- protocols may be implemented in groups other than Z p * and a particularly robust group is the group of points on an elliptic curve over a finite field.
- An example of such an implementation is set out below as protocol 1"'.
Abstract
Description
- The present invention relates to key agreement protocols for transfer and authentication of encryption keys.
- To retain privacy during the exchange of information it is well known to encrypt data using a key. The key must be chosen so that the correspondents are able to encrypt and decrypt messages but such that an interceptor cannot determine the contents of the message.
- In a secret key cryptographic protocol, the correspondents share a common key that is secret to them. This requires the key to be agreed upon between the correspondents and for provision to be made to maintain the secrecy of the key and provide for change of the key should the underlying security be compromised.
- Public key cryptographic protocols were first proposed in 1976 by Diffie-Hellman and utilized a public key made available to all potential correspondents and a private key known only to the intended recipient. The public and private keys are related such that a message encrypted with the public key of a recipient can be readily decrypted with the private key but the private key cannot be derived from the knowledge of the plaintext, ciphertext and public key.
- Key establishment is the process by which two (or more) parties establish a shared secret key, called the session key. The session key is subsequently used to achieve some cryptographic goal, such as privacy. There are two kinds of key agreement protocol; key transport protocols in which a key is created by one party and securely transmitted to the second party; and key agreement protocols, in which both parties contribute information which jointly establish the shared secret key. The number of message exchanges required between the parties is called the number of passes. A key establishment protocol is said to provide implicit key authentication (or simply key authentication) if one party is assured that no other party aside from a specially identified second party may learn the value of the session key. The property of implicit key authentication does not necessarily mean that the second party actually possesses the session key. A key establishment protocol is said to provide key confirmation if one party is assured that a specially identified second party actually has possession of a particular session key. If the authentication is provided to both parties involved in the protocol, then the key authentication is said to be mutual; if provided to only one party, the authentication is said to be unilateral.
- There are various prior proposals which claim to provide implicit key authentication.
- Examples include the Nyberg-Rueppel one-pass protocol and the Matsumoto-Takashima-Imai (MTI) and the Goss and Yacobi two-pass protocols for key agreement. The Nyberg-Rueppel protocol and the MTI protocol are described in EP 0 639 907 and MATSUMOTO, T., TAKASHIMA, Y., and IMAI, H.: On Seeking Smart Public-Key-Distribution Systems, The Transactions of the IECE of Japan, E69: 99-106, 1986. The Goss protocol is described in U.S. Patent No. 4,956,865. The Yacobi protocol is described in Y. Yacobi, "A Key Distribution Paradox," Advances in Cryptology, Crypto '90, Lecture Notes in Computer Science 537, Springer-Verlag, 1991, pp. 268 to 273.
- The prior proposals ensure that transmissions between correspondents to establish a common key are secure and that an interloper cannot retrieve the session key and decrypt the ciphertext. In this way security for sensitive transactions such as transfer of funds is provided.
- For example, the MTI/AO key agreement protocol establishes a shared secret K, known to the two correspondents, in the following manner:-
- 1. During initial, one-time setup, key generation and publication is undertaken by selecting and publishing an appropriate system prime p and generator
- 2. The protocol requires the exchange of the following messages.
The values of x and y remain secure during such transmissions as it is impractical to determine the exponent even when the value of a and the exponentiation is known provided of course that p is chosen sufficiently large. - 3. To implement the protocol the following steps are performed each time a shared key is required.
- (a) A chooses a random integer x,1 ≤x≤p-2, and sends B message (1) i.e. αx mod p.
- (b) B chooses a random integer y, 1≤y≤p-2, and sends A message (2) i.e. αy mod p.
- (c) A computes the key K = (αy)azB x mod p.
- (d) B computes the key K = (αx)bZA y mod p.
- (e) Both share the key K = αbx+ay.
- In order to compute the key K, A must use his secret key a and the random integer x, both of which are known only to him. Similarly B must use her secret key b and random integer y to compute the session key K. Provided the secret keys a,b remain uncompromised, an interloper cannot generate a session key identical to the other correspondent. Accordingly, any ciphertext will not be decipherable by both correspondents.
- EP 0 639 907 describes a key agreement method. User A has a private key agreement key sA. User B has a public key agreement key kB corresponding to the private key agreement key sB through the rule kB = g-S
B mod p . - As such this and related protocols have been considered satisfactory for key establishment and resistant to conventional eavesdropping or man-in-the-middle attacks.
- In some circumstances it may be advantageous for an adversary to mislead one correspondent as to the true identity of the other correspondent.
- In such an attack an active adversary or interloper E modifies messages exchanged between A and B, with the result that B believes that he shares a key K with E while A believes that she shares the same key K with B. Even though E does not learn the value of K the misinformation as to the identity of the correspondents may be useful.
- A practical scenario where such an attack may be launched successfully is the following. Suppose that B is a bank branch and A is an account holder. Certificates are issued by the bank headquarters and within the certificate is the account information of the holder. Suppose that the protocol for electronic deposit of funds is to exchange a key with a bank branch via a mutually authenticated key agreement. Once B has authenticated the transmitting entity, encrypted funds are deposited to the account number in the certificate. If no further authentication is done in the encrypted deposit message (which might be the case to save bandwidth) then the deposit will be made to E's account.
- It is therefore an object of the present invention to provide a protocol in which the above disadvantages are obviated or mitigated.
- According therefore to the present invention there is provided a method of authenticating a key established between a pair of correspondents as described in the appended claims.
- Thus although the interloper E can substitute her public key pE = αae in the transmission as part of the message, B will use pE rather than pA when authenticating the message. Accordingly the computed and transmitted values of the exponential functions will not correspond.
- Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which:-
- Figure 1 is a schematic representation of a data communication system.
- Referring therefore to Figure 1, a pair of correspondents, 10,12, denoted as correspondent A and correspondent B, exchange information over a
communication channel 14. Acryptographic unit correspondents channel 14. Akey 20 is associated with each of thecryptographic units unit respective correspondent channel 14. - In operation, a message generated by correspondent A, 10, is encrypted by the
unit 16 with the key 20 and transmitted as ciphertext overchannel 14 to theunit 18. - The key 20 operates upon the ciphertext in the
unit 18 to generate a plaintext message for the correspondent B, 12. Provided thekeys 20 correspond, the message received by thecorrespondent 12 will be that sent by thecorrespondent 10. - In order for the system shown in Figure 1 to operate it is necessary for the
keys 20 to be identical and therefore a key agreement protocol is established that allows the transfer of information in a public manner to establish the identical keys. A number of protocols are available for such key generation and are variant of the Diffie-Hellman key exchange. Their purpose is for parties A and B to establish a secret session key K. - The system parameters for these protocols are a prime number p and a generator a of the multiplicative group
- In each example below it is assumed that, an interloper E wishes to have messages from A identified as having originated from E herself. To accomplish this, E selects a random integer e, 1≤e≤p-2, computes pE=(pA)e=αae mod p, and gets this certified as her public key. E does not know the exponent ae, although she knows e. By substituting textE for textA, the correspondent B will assume that the message originates from E rather than A and use E's public key to generate the session key K. E also intercepts the message from B and uses his secret random integer e to modify its contents. A will then use that information to generate the same session key allowing A to communicate with B.
- To avoid interloper E convincing B that he is communicating with E, the following protocol is adapted.
- The purpose of the protocol is for parties A and B to establish a session key K. The protocols exemplified are role-symmetric and non-interactive. The protocols labelled First Protocol, Modified First Protocol, Second Protocol, Third Protocol, and Key Transport Protocol are presented to show general concepts, but these particular protocols are not part of the invention presently claimed.
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- 1. A picks a random integer x,1≤x≤p-2, and computes rA = αx and a signature sA = x - rAa mod (p - 1). A sends {rA,SA,textA} to B.
- 2. B picks a random integer y,1≤y≤p-2, and computes rB = αy and a signature sB = y - rBb mod (p -1). B sends {rB,sB,textB} to A.
- 3. A computes αs
B (pB)rB and verifies that this is equal to rB. A computes the session key - 4. B computes αs
A (PA)rA and verifies that this is equal to rA. B computes the sessin key - Should E replace text A with textE, B will compute αs
B (pE ) rA which will not correspond with the transmitted value of rA. B will thus be alerted to the interloper E and will proceed to initiate another session key. - One draw back of the first protocol is that it does not offer perfect forward secrecy. That is, if an adversary leams the long-term private key a of party A, then the adversary can deduce all of A's past session keys. The property of perfect forward secrecy can be achieved by modifying
Protocol 1 in the following way. - In
step 1, A also sends α x1 to B, where x1 is a second random integer generated by A. Similarly, in step 2 above, B also sends α y1 to A, where y1 is a random integer. A and B now compute the key K =αxy ⊕α x1 y1 . - Another drawback of the first protocol is that if an adversary learns the private random integer x of A, then the adversary can deduce the long-term private key a of party A from the equation sA = x -rAa mod p - 1. This drawback is primarily theoretical in nature since a well designed implementation of the protocol will prevent the private integers from being disclosed.
- A second protocol set out below addresses these two drawbacks.
- 1. A picks a random integer x,1≤x≤p-2, and computes (pB)x,αx and a signature sA = x + a(pB)x mod (p-1). A sends {αx,sA,textA} to B.
- 2. B picks a random integer y,1≤y≤p-2, and computes (pA)y,αy and a signature sB = y + b(pA)y mod (p-1). B sends {αy,sB,textB} to A.
- 3. A computes (αy)a and verifies that αS
B (pB ) -ααy = αy .
A then computes session key K = αay(pB)x. - 4. B computes (αx)b and verifies that αs
A (pA ) -αbx = αx. A then computes session key K = αbx(pA)y. - The second protocol improves upon the first protocol in the sense that if offers perfect forward secrecy. While it is still the case that disclosure of a private random integer x allows an adversary to learn the private key a, this will not be a problem in practice because A can destroy x as soon as she uses it in
step 1 of the protocol. - If A does not have an authenticated copy of B's public key then B has to transmit a certified copy of his key to B at the beginning of the protocol. In this case, the second protocol is a three-pass protocol.
- The quantity sA serves as A's signature on the value αx This signature has the novel property that it can only be verified by party B. This idea can be generalized to all ElGamal-like signatures schemes.
- The first and second protocols above can be modified to improve the bandwidth requirements and computational efficiency of the key agreement. The modified protocols are described below as Protocol 1' and Protocol 2'. In each case, A and B will share the common key αs
A sB . -
- 1. A picks a random integer x, 1≤x≤p-2, and computes rA = αa and sA = x + rAaαa mod (p-1). A sends {rA, textA} to B.
- 2. B picks a random integer y, 1≤y≤p-2, and computes rB = αy and sB = y + rBbαb mod (p-1). B sends {rB, textB} to A.
- 3. A computes K=(rB (pB ) rBαb ) s
A which is equivalent to αsA sB . - 4. B computes K=(rA(pA ) rAαa ) s
B which is also equivalent to αsA sB . - A and B thus share the common key but it will be noted that the signatures sA and sB need not be transmitted.
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- 1. A picks a random integer x, 1≤x≤p-2, and computes (pB)x, αx and sA= x + a(pB)x mod (p-1). A sends {αx, textA} to B.
- 2. B picks a random integer y, 1≤y≤p-2, and computes (PA)y, αy and sB = y + b(pA)y mod (p-1). B sends {αy, textB} to A.
- 3. A computes (αy)a and K=(αy(pB) α bαxy )S
A . i.e. αsA sB . - 4. B computes (αx)b and K=(αx(pA) α aαbx )s
B . i.e. αsA sB . - Again therefore the transmission of sA and sB is avoided.
- A further protocol is available for parties A and B to establish a session key K.
-
- 1. A picks two random integers x, x1, 1≤x,x1≤p-2, and computes r x
1 =α x1 , rA=αx and1 , textA } to B. - 2. B picks two random integers y, y1, 1≤y,y1≤p-2, and computes r y
1 =α y1 , rB=αy and1 , textB } to A. - 3. A computes
1 ) x1 =α x1 y1 . - 4. B computes
1 ) y1 =α x1 y1 . - In these protocols, (rA, sA) can be thought of as the signature of r x
1 with the property that only A can sign the message r x1 . - The protocols described above permit the establishment and authentication of a session key K. It is also desirable to establish a protocol in which permits A to transport a session key K to party B. Such a protocol is exemplified below.
- 1. A picks a random integer x, 1≤x≤p-2 and computes rA = αx and a signature SA = x-rAaαa mod (p-1). A computes session key K = (PB)x and sends {rA, SA, textA} to B.
- 2. B computes αs
A (pA)rA αa and verifies that this quantity is equal to rA. B computes session key K = (rA)b. - The above protocol may be modified to reduce the bandwidth by avoiding the need to transmit the signature SA as follows:
- 1. A picks a random integer x, 1≤x≤p-2, and computes rA = αx and sA = x - rAaαa mod (p-1). A computes K=(PB)s
A and sends {rA, textA) to B. - 2. B computes K=(αx(pA)-r
A αa ) b=αbsA . - All one-pass key transport protocols have the following problem of replay. Suppose that a one-pass key transport protocol is used to transmit a session key K from A to B as well as some text encrypted with the session key K. Suppose that E records the transmission from A to B. If E can at a later time gain access to B's decryption machine (but not the internal contents of the machine, such as B's private key), then, by replaying the transmission to the machine, E can recover the original text. (In this scenario, E does not learn the session key K).
- This replay attack can be foiled by usual methods, such as the use of timestamps. There are, however, some practical situations when B has limited computational resources, in which it is more suitable at the beginning of each session, for B to transmit a random bit string k to A. The session key that is used to encrypt the text is then k ⊕ K, i.e. k XOR'd with K.
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- All the protocols discussed above have been described in the setting of the multiplicative group
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- In the above protocols, a signature component of the general form SA = x + ra.a.αa has been used.
- The protocols may be modified to use a simpler signature component of the general form sA = x + ra.a without jeopardizing the security.
- Examples of such protocols will be described below using the same notation although it will be understood that the protocols could be expressed in alternative notation if preferred.
-
- p is a prime number,
- a is a generator of
- a and b are party A's and B's respective long-term private key,
- αa mod p is party A's long-term public key,
- αb mod p is party B's long-term public key,
- x is a random integer selected by A as a short-term private key,
- ra = αx mod p is party A's short-term public key,
- y is a random integer selected by B as a short-term private key,
- rb = αy mod p is party B's short-term public key,
-
-
- To implement the protocol,
- 1. A sends ra to B.
- 2. B sends rB to A.
- 3. A computes
- 4. A computes the session key K where
- 5. B computes
- 6. B computes the session key K where
- 7. The shared secret is αs
B sA mod p. - In this protocol, the bandwidth requirements are again reduced by the signature components combine the short and long-term keys of the correspondent to inhibit an attack by an interloper.
-
- A's and B's public keys will be of the form ga, gb respectively and the short-term keys ra, rb will be of the form gx, gy.
- The signature components sA, sB are computed mod q and the session key K computed mod q as before. The shared secret is then gs
A sB mod p. -
- The following notation is used:
- E is an elliptic curve defined over Fq,
- P is a point of prime order n in E(Fq),
- da (1<da<n-1) is party A's long-term private key,
- db (1<db<n-1) is party B's long-term private key,
- Qa = daP is party A's long-term public key,
- Qb=dbP is party B's long-term public key,
- k(1<k<n-1) is party A's short-term private key,
- ra= kP is party A's short-term public key,
- m (1<m<n-1) is party B's short-term private key,
- rb = mP is party B's short-term public key,
-
- To implement the protocol:
- 1. A sends ra to B.
- 2. B sends rb to A.
- 3. A computes
- 4. A computes the session key K where
- 5. B computes
- 6. B computes the session key K where
- 7. The shared secret is sasbP.
- Again, it will be noted that it is not necessary to send the signature components sA, sB between the correspondent but the short and long-term keys of the correspondents are combined by the form of the components. (It will be appreciated that the notation m has been substituted for x,y in the previous examples to avoid confusion with the co-ordinate (x,y) of the points on the curve).
Claims (27)
- A method of authenticating a key (20) established between a pair of correspondents (10, 12) A, B in a public key data communication system to permit exchange of information (16, 18) therebetween over a communication channel (14), each of said correspondents (10, 12) having a respective private key and a public key derived from a generator and respective ones of said private keys , said method including the steps of:i) a first of said correspondents A (10) selecting a first random integer and exponentiating a function including said generator to a power to provide a first exponentiated function ra;ii) said first correspondent A (10) generating a first signature SA from said random integer and an integeriii) said first correspondent A (10) forwarding to a second correspondent B a message derived from said first exponentiated function ra;iv) said correspondent B (12) selecting a second random integer and exponentiating a function including said generator to a power to provide a second exponentiated function rb and generating a signature sB obtained from said second random integer and an integerv) said second correspondent B (12) forwarding a message to said first correspondent A derived from said second exponential function rb;vi) each of said correspondents (10,12) constructing a session key K (20) by exponentiating information made public by the other correspondent with the signature that is private to themselves.
- A method of claim 1 wherein said message forwarded by said first correspondent (10) includes an identification of the first correspondent.
- A method according to claim 1 or 2 wherein said message forwarded by said second correspondent (12) includes an identification of said second correspondent (12).
- A method according to claim 1,2 or 3 wherein said first function including said generator is said generator itself.
- A method according to any preceding claim wherein said second function including said generator is said generator itself.
- A method according to any preceding claim wherein said first function including said generator includes the public key of said second correspondent (12).
- A method according to any preceding claim wherein said second function including said generator includes the public key of said first correspondent (10).
- A method according to any preceding claim wherein said signature generated by a respective one of the correspondents (10, 12) combines the random integer, exponentiated function and private key of that one correspondent (10, 12).
- A method according to any preceding claim wherein said information made public by another correspondent (10, 12) combines the public key, exponentiated function, and integer of that one correspondent (10,12).
- A method according to claim 9 wherein the public key and exponentiated function are multiplied and the resultant value is exponentiated with the integer.
- A method according to claim 11 wherein the shared secret is αs
A sB . - A method according to any one of claims 1 to 10, wherein said generator α is a point P of order n on an elliptic curve defined over a finite field Fq, exponentiation is performed by scalar multiplication on said elliptic curve.
- A method according to claim 19, wherein said integers are bit strings of coordinates of said exponentiated functions.
- A method according to claim 20, wherein said bit strings are least significant bits of the x coordinates of said exponentiated functions.
- A method according to claim 21, wherein said bit strings are the 80 least significant bits of the x coordintes of said exponentiated functions.
- A method according to any one of claims 19 to 26, wherein the point P is of prime order.
Priority Applications (10)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
EP10186318.1A EP2315390B1 (en) | 1996-10-18 | 1996-10-18 | Key agreement and transport protocol with implicit signatures |
DE69637956T DE69637956D1 (en) | 1996-10-18 | 1996-10-18 | Session key generation method with implicit signatures |
EP07000020A EP1768300B1 (en) | 1996-10-18 | 1996-10-18 | Key agreement and transport protocol with implicit signatures |
EP09162881.8A EP2104268B1 (en) | 1996-10-18 | 1996-10-18 | Key agreement and transport protocol with implicit signatures |
EP10186315.7A EP2315389B1 (en) | 1996-10-18 | 1996-10-18 | Key agreement and transport protocol with implicit signatures |
EP10186321.5A EP2315391B1 (en) | 1996-10-18 | 1996-10-18 | Key agreement and transport protocol with implicit signatures |
HK07109632.8A HK1101628A1 (en) | 1996-10-18 | 2007-09-04 | Key agreement and transport protocol with implicit signatures |
HK11111420.4A HK1157524A1 (en) | 1996-10-18 | 2011-10-24 | Key agreement and transport protocol with implicit signatures |
HK11111605.1A HK1157529A1 (en) | 1996-10-18 | 2011-10-27 | Key agreement and transport protocol with implicit signatures |
HK11111600.6A HK1157528A1 (en) | 1996-10-18 | 2011-10-27 | Key agreement and transport protocol with implicit signatures |
Applications Claiming Priority (3)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
US42671295A | 1995-04-21 | 1995-04-21 | |
US08/442,833 US5761305A (en) | 1995-04-21 | 1995-05-17 | Key agreement and transport protocol with implicit signatures |
PCT/US1996/016608 WO1998018234A1 (en) | 1995-04-21 | 1996-10-18 | Key agreement and transport protocol with implicit signatures |
Related Child Applications (5)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
EP10186321.5A Division EP2315391B1 (en) | 1996-10-18 | 1996-10-18 | Key agreement and transport protocol with implicit signatures |
EP07000020A Division EP1768300B1 (en) | 1996-10-18 | 1996-10-18 | Key agreement and transport protocol with implicit signatures |
EP09162881.8A Division EP2104268B1 (en) | 1996-10-18 | 1996-10-18 | Key agreement and transport protocol with implicit signatures |
EP10186318.1A Division EP2315390B1 (en) | 1996-10-18 | 1996-10-18 | Key agreement and transport protocol with implicit signatures |
EP10186315.7A Division EP2315389B1 (en) | 1996-10-18 | 1996-10-18 | Key agreement and transport protocol with implicit signatures |
Publications (2)
Publication Number | Publication Date |
---|---|
EP0873617A1 EP0873617A1 (en) | 1998-10-28 |
EP0873617B1 true EP0873617B1 (en) | 2007-01-03 |
Family
ID=27027154
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
EP96944186A Expired - Lifetime EP0873617B1 (en) | 1995-04-21 | 1996-10-18 | Key agreement and transport protocol with implicit signatures |
Country Status (7)
Country | Link |
---|---|
US (1) | US5761305A (en) |
EP (1) | EP0873617B1 (en) |
JP (1) | JP4384728B2 (en) |
AU (1) | AU1405797A (en) |
CA (1) | CA2237688C (en) |
DE (1) | DE69636815T2 (en) |
WO (1) | WO1998018234A1 (en) |
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Also Published As
Publication number | Publication date |
---|---|
US5761305A (en) | 1998-06-02 |
DE69636815T2 (en) | 2007-11-08 |
AU1405797A (en) | 1998-05-15 |
WO1998018234A1 (en) | 1998-04-30 |
JP4384728B2 (en) | 2009-12-16 |
CA2237688A1 (en) | 1998-04-30 |
CA2237688C (en) | 2007-07-24 |
DE69636815D1 (en) | 2007-02-15 |
JP2000502553A (en) | 2000-02-29 |
EP0873617A1 (en) | 1998-10-28 |
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